Terrestrially observable displays from space

ABSTRACT

A nanosatellite with an illumination element, and an arrangement of nanosatellites in low earth orbit (LEO) arranged to controllably apply their illumination to be visible on the ground. The nanosatellites may be coordinated to provide illumination events visible on the ground at particular locations and particular times.

RELATED APPLICATIONS

This application claims the benefit of priority under 35 USC 119(e) ofU.S. Provisional Patent Application Nos. 62/126,860 filed on Mar. 2,2015 and 62/127,351 filed on Mar. 3, 2015. The contents of the aboveapplications are all incorporated by reference as if fully set forthherein in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to displaysin space that are observable from the ground, and, more particularly,but not exclusively, to a satellite and control arrangement to providesuch a display.

There have been proposals for creating signs in space that would bevisible from the ground, for example—for advertising purposes.

Known solutions require large satellites and expensive (heavy)deployment mechanisms and resources.

An example of the existing art is US Patent Application No. 2012/0223189published Sep. 12, 2012 of Hitoshi Kuninaka, which relates to anapparatus and method for generating a flash of light towards the earthfrom a satellite. The suggested method is to use reflection of sunlightand thus save on the high costs associated with allowing a batteryoperated satellite to produce enough light to be viewed from earth.Occasionally, an antenna reflects sunlight directly down at Earth,creating a predictable and quickly moving illuminated spot on thesurface below of about 10 km (6.2 mi) diameter. To an observer thislooks like a bright flash, or flare in the sky, with a duration of a fewseconds. Reflection of sun light from satellites is carried out in asingle stage, and depends on orbit, location, and angular state ofsatellites with respect to points of interest on earth.

The suggestion is based on the concept of the so-called Iridium flaresthat can be seen on earth from the low earth orbit Iridium satelliteconstellation. An array of satellites is mentioned although there is nodiscussion of the practicality of having any large number of suitablyequipped satellites.

SUMMARY OF THE INVENTION

The present embodiments provide an array of nanosatellites for formingthe sign. The present embodiments may provide display of changing signsover different territories and time windows. The nanosatellites allowfor multiple illumination points in a matrix to provide letters andwords, and yet can be conveniently launched from a single launchmission.

According to an aspect of some embodiments of the present inventionthere is provided a nanosatellite carrying an illumination element, theillumination element configured for sustained external illumination. Theillumination element may comprise a light emitting diode, battery powersufficient to illuminate said lamp for at least a minute, andphotovoltaic recharging ability. The nanosatellites may have controlability to position themselves with other nanosatellites in apredetermined array, and/or to operate said lamp in coordination withsaid other nanosatellites, thereby to provide an illumination eventvisible from the ground.

The nanosatellites may have directional capability to point saidillumination element towards ground.

In an embodiment, the illumination element is releasable.

The nanosatellite may have control ability to position itself with othernanosatellites in an array, and to release said illumination element incoordination with said other nanosatellites, thereby to provide anillumination event visible from the ground.

The nanosatellite may position itself with other nanosatellites in anarray and carry a wire connected to at least one other of saidnanosatellites in said array, at least one additional illuminationelement being located on said wire.

The nanosatellite may alternatively position itself with othernanosatellites in an array while holding a sheet connected to others ofsaid nanosatellites in said array.

In an embodiment, said illumination element is a mirror, said mirrorbeing directable to deflect focused light from another satellite sourceat the ground.

According to a second aspect of the present invention there is provideda ground-based satellite control system configured to control a firstarray of nanosatellites in low earth orbit to produce an illuminationevent visible at a predetermined ground location.

In an embodiment, said first array comprises at least one matrix ofillumination positions, said matrix sized such that controllableswitching of said illumination positions forms letters visible from saidpredetermined ground location.

The control system may switch said illumination positions to change saidletters during said illumination event.

The control system may control a second array of nanosatellites to shinefocused light onto said first array for redirection by said first array.

The control system may schedule said illumination event along with otherillumination events in accordance with available battery power on boardsaid nanosatellites and in accordance with recharging opportunities ofan orbit of said nanosatellites.

The control system may add additional available nanosatellites to saidfirst array according to requirements of an illumination event.

The control system may control an illumination event to fit in with aduration during which said first array is visible over saidpredetermined ground location.

According to a third aspect of the present invention there is provided amethod of controlling nanosatellites comprising:

arranging said nanosatellites in a first array in low earth orbit; and

operating illumination elements in association with said nanosatellitesto produce an illumination event visible at a predetermined groundlocation.

In embodiments, the illumination element, and the event as a whole, maybe in a frequency range outside of a visible part of the spectrum, suchas infra-red.

A further aspect of the present invention may provide a method ofcontrolling unmanned aerial vehicles comprising:

arranging said unmanned aerial vehicles in a flying formation; and

operating illumination elements in association with said unmanned aerialvehicles to produce an illumination event visible at a predeterminedground location.

A ground-based control system may control a first array of unmannedaerial vehicles in flight to produce an illumination event visible at apredetermined ground location.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data.

Optionally, a network connection is provided as well. A display and/or auser input device such as a keyboard or mouse are optionally provided aswell.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1 is a simplified diagram illustrating a battery powerednanosatellite according to an embodiment of the present invention;

FIG. 2 is a simplified diagram illustrating a 4×5 nanosatellite arrayfor use in the present embodiments;

FIG. 3 is a simplified diagram showing the letter ‘c’ being displayed ona 4×4 array of nanosatellites according to an embodiment of the presentinvention;

FIG. 4 is a simplified diagram illustrating a ground station that mightbe used to control nanosatellites according to the present embodiments,or relay control signals to the nanosatellites;

FIG. 5 is a simplified flow chart illustrating a procedure for arrangingnanosatellites as an array in low earth orbit and controlling thesatellites to provide a display to be viewed at a pre-arranged locationon the ground;

FIG. 6 is a simplified diagram illustrating two satellites with a wirebetween them and an illumination element mounted on the wire accordingto an embodiment of the present invention;

FIG. 7 is a simplified diagram illustrating four satellites with a sheetsuspended between them according to an embodiment of the presentinvention;

FIG. 8 is a simplified diagram illustrating two satellites with a lensand mirror for redirecting sunlight to illuminate an event according toembodiments of the present invention; and

FIG. 9 is a simplified diagram illustrating a module for scheduling andcontrolling events according to an embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to displaysin space that are observable from the ground, and, more particularly,but not exclusively, to a satellite and control arrangement to providesuch a display.

The present inventions relate to the use of nanosatellites operating inan array to form a sign in space that may be observed on the ground.While the light from any individual satellite may not be meaningful, thearray as a whole forms signs that are within the resolving power of theground based human eye.

The present embodiments address issues relating to the way in whichsigns may be formed in light of constraints on available energy, andthese include ways to exploit sunlight.

Forming a sign in space is a challenging undertaking, and the presentembodiments may provide a practical and economic solution compared tothe existing art. Nano-satellite technology is now capable of providingformation flight of multiple nanosatellites.

A possible commercial application is advertising. Other applicationsinclude major national celebrations, where for example the sign could becoordinated with a fireworks display, and disaster warnings.

The present embodiments may rely on advances in nano-satellite controland thrusters, and may utilize a multitude of satellites. Thus, a swarmof satellites, flying in formation with about 200 m between adjacentsatellites, with light-power control, may display a pattern over aterritory and at a time as specified.

In one embodiment, more lengthy messages may be scrolled over thedisplay just as with a conventional dot-matrix-type display. Droppeddisplay elements that shine but cannot maneuver is another option.

The challenge of using nanosatellites is that human low light orscotopic vision is sensitive to less than 100 visible (say green)photons per second, or light power of ˜3×10⁻¹⁰ erg/s or 3×10⁻¹⁷ Watt. ALED (Light Emitting Diode) may be connected to a satellite battery. ALED is a very efficient light source so that a battery is sufficient toproduce the needed power output to provide sufficient lighting power forthe short time interval when the satellite is in view of the location ofinterest. After a few minutes the satellite moves out of sight, and thebattery can be recharged. Furthermore LED lights do not take up muchspace.

At a distance of d=600 km above the ground, which is a stable LEO orbit,the satellite may provide non-beamed light. Two Watts of light aresufficient for night visibility in good weather and visibilityconditions.

Reflective panels may be added to the satellite. At certain observingangles, the reflective panels of the satellite may augment theelectrically produced luminosity.

A mother ship with extra satellites and fuel may be provided to managethe swarm of nano satellites.

A system of lenses and mirrors can be used for early evening displays todivert sun light onto the system, and thus solve the problem of poweringthe display, at the cost of losing most of the night.

Very large sheets can be supported by the satellites and may useillumination powered by thin photovoltaic illumination elements, orprojection, or may use reflection by lenses and mirrors.

The present embodiments provide interesting possibilities foradvertising and publicity in general, and can be used to provide instantwarnings over large geographical areas, for example to warn of atsunami. The embodiments can be used as part of a national event, toprovide a non-standard greeting, and could even be used for globalcomputer games.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

Referring now to the drawings, FIG. 1 illustrates a nanosatellite 10carrying an illumination element 12. The illumination element is anelement that provides sustained external illumination, as opposed to aflash, which provides momentary illumination or an indicator light whichdoes not illuminate anything but merely indicates that certain equipmentis working or in a given state.

Illumination element 12 may be a light emitting diode (LED), inparticular an array of short wavelength, white or blue, LEDs, as theseprovide considerable illumination for minimal power and are reasonablyrobust to space flight. The nanosatellite may contain a battery 14 topower the illumination, and the available battery power may besufficient to illuminate the lamp for at least a minute. Satellite 10may also include photovoltaic recharging ability.

Nanosatellite 10 has a weight of between around 1 kg and 10 kg, and atypical size of a nanosatellite is a side length of 100 mm, or severalunits thereof, as indicated by arrow 18. An advantage of usingnanosatellites is that they are on the one hand large enough to providesufficient illumination for a reasonable amount of time and on the otherhand are small enough that launching enough satellites to form an arrayis feasible. In fact several arrays can be launched using a singlemedium sized rocket.

Herein, reference to a nanosatellite includes satellites that aresmaller than conventionally recognized as a nanosatellites as long asthey are able to carry an illumination element which, when placed in anarray with other nanosatellites can provide illumination visible on theground from LEO.

Reference is now made to FIG. 2, which is a simplified schematic diagramshowing an array of nanosatellites in orbit according to a preferredembodiment of the present invention. The nanosatellites may have controlability to position itself with other nanosatellites in predeterminedarray. The control ability provides the nanosatellites with the abilityto maneuver themselves in orbit and therefore maintain themselves in anarray. It is noted that changing orbit is very expensive in terms ofenergy but minor changes for stability or maintenance of an array aremore suitable for battery and solar powered satellites. The command forcontrolling the array may come from another of the nanosatellites, froma mother satellite or from a ground station.

The array of nanosatellites may be controlled to provide sufficientdistance between the satellites to accord with the eye's ability toresolve shapes from the ground, to ensure that particular shapes orletters are visible during the event. FIG. 3 illustrates how certain ofthe nanosatellites in the array of FIG. 2 can have their illuminationswitched on to form a letter or character, in this case the letter ‘c’.

The individual satellites may have directional capability to enable themto direct the illumination element towards the ground.

In one embodiment the illumination element is releasable. For example aflare carrying component could be released from the satellite and lit.As long as all the satellites in the matrix release elements at the sametime the matrix formation will be retained by the released elements.Upon release the flare or other illumination may be lit.

The satellites in the matrix are controlled to coordinate illuminationand to coordinate their flight.

In one embodiment, the nanosatellites in the array suspend anelectrically conducting wire between them, and additional illuminationelements are mounted on the wire. The result is that a matrix can havemore illumination points than the number of available nanosatellites.

In a further embodiment, the nanosatellites may hold up a sheet. Lightmay be projected onto the sheet to form a screen. As will be discussedbelow, for the projected image to be visible on the ground the sheet mayhave to be of the order of magnitude of a square kilometer in size.

In an embodiment, the illumination element 14 is a mirror, which can bedirected to deflect focused light from another satellite source at theground. Light can be deflected from the sun or focused light can beprovided from a second array of satellites, as will be discussed ingreater detail below.

Reference is now made to FIG. 4, which illustrates a ground station 40for controlling the satellites and satellite array or arrays. Thesatellites and the illumination events are typically controlled at aground station such as illustrated ground station 40. The ground-basedsatellite control system may control arrays of nanosatellites in lowearth orbit to produce the illumination events to be visible at givenground locations at given times.

The array being controlled may comprise a matrix of illuminationpositions as in FIG. 2 discussed above. The matrix may be of a size toallow controllable switching of the illumination positions to formletters visible from the ground during the event. The distance betweenthe satellites determines the matrix size. The letters may be switchedduring the illumination event to change the letters as needed. Typicallythe satellites are visible at a given location for one or two minutes,allowing the letters to change several times or to give the appearanceof scrolling the letters.

The ground control may control a second array of nanosatellites to shinefocused light onto the first array for redirection by the first arraytowards the ground. The second array may be equipped with lenses tofocus sunlight towards the first array, now in darkness, so as to enablethe first array to reflect the sunlight towards the ground.

The satellite array is ideally used in the most efficient way possible.Events are only visible at night, and the batteries can only recharge inthe sunlight, so that battery capacity determines how many events may beaccommodated in a single orbit. If the batteries can provide about twominutes of illumination then only a single event can be accommodated perorbit. The ground control may schedule multiple illumination events tomake the most efficient use of the available battery power and to avoida schedule of events that is beyond the available battery power.

Now in fact during an event there may be some array positions, which areunused or only used very lightly. So it may often be possible togenerate a smaller but still operational matrix for a subsequent eventeven though some satellites may have run out of power. Thus the groundcontrol may track the different satellites to determine the batterycharge on each satellite. Furthermore, batteries tend to deteriorateover a lifetime of charging and recharging, and the lifetime effect maybe taken into account by the ground controller.

The ground controller may also add additional available nanosatellitesto the array as needed by a given event.

Typically, events require the satellites to be visible at a particulartime. However, satellites are limited by their orbits, and groundcontrol can control the satellites to illuminate while they are visiblefrom the required location. Furthermore, the ground control canreschedule a given event for a later orbit as needed, in which case theground location needs to be notified.

Reference is now made to FIG. 5, which is a simplified flow chartillustrating a procedure for providing illumination events. Thenanosatellites are placed in low earth orbit 50 and are controlled tofly together in formation 52, typically as an array. The satellites arethen controlled to pass 54 within view of an event location and then theevent itself consists of the satellites switching on illumination in acontrolled manner while in view of the event location and during thetime of the event.

As discussed, the array provides a matrix of illumination positions, andcontrollable switching of the illumination positions forms letters,logos, designs and even images, which may then be viewed from the groundlocation. Depending on the number of satellites available, slogans canbe displayed altogether or one word at a time, and words can bedisplayed altogether or one letter at a time.

As mentioned, illumination may be via illumination elements on thesatellites themselves, or by dropped elements that are released from thesatellites and switch themselves on or by use of reflected light.

The embodiments are now considered in greater detail. Today, informationaddressed to the public, from advertisements to greetings to publicinformation to warnings and everything in between, is ubiquitous and ismade available through all kinds of available media.

The present embodiments may provide an additional medium to address thepublic. The idea is to display the information from space in such a wayas to be visible on earth. The present embodiments may use satellites,typically in Low Earth Orbit (LEO) for creating a visual effect of asign. The term low Earth orbit (LEO) is an orbit around Earth with analtitude between 160 kilometers (99 mi), (orbital period of about 88minutes), and 2,000 kilometers (1,200 mi) (about 127 minutes). In orderto avoid drag from atmospheric gases, an altitude of at least 300 km isgenerally chosen for satellites. Higher orbits on the other hand wouldrequire greater illumination in order to be visible.

The present embodiments may use multiple satellites, in arrays orswarms, to create letters, words and images. The individual satellitesmay be nanosatellites, whose movement is controlled together, eachhaving a light, typically an LED light and acting as a pixel of thearray. The term “nanosatellite” or “nanosat” is applied to an artificialsatellite with a wet mass, meaning the mass including fuel, of between 1and 10 kg (2.2 and 22.0 lb). Designs and proposed designs of these typesmay be launched individually, or they may have multiple nanosatellitesworking together or in formation, in which case, sometimes the term“satellite swarm” or “fractionated spacecraft” may be applied. Asdiscussed herein, some designs require a larger mother satellite forcommunication with ground controllers or for launching and docking withnanosatellites.

With continued advances in the miniaturization and capability increaseof electronic technology and the use of satellite constellations,nanosatellites are increasingly capable of performing commercialmissions that previously required microsatellites.

Satellite classifications according to size are nanosatellites as above,picosatellites, which have a mass of between 0.1 kg and 1 kg,femtosatellites, which have a mass below 0.1 kg and usually require amother ship, microsatellites, whose weight lies between 10 kg and 100kg, and small satellites which lie between 100 and 500 kg.

An example of a nano-satellite is the CubeSat, shown in FIG. 1, which iscurrently widely used for space research. The CubeSat specificationdefines a volume of exactly one litre (103 cm3), with a mass of no morethan 1.33 kilograms, and typically uses commercial off-the-shelfcomponents for its electronics. Several companies build nanosatellitesto the CubeSat specification, such as large-satellite-makerBoeing.

A further example of a nano-satellite is the Mepsi, made by NASA.

With the development of small, efficient propulsion thrusters, as wellas algorithms for maintaining relative positioning of satellites overtime, it is now possible to achieve nanosatellite arrays and swarmingeffects as described herein.

A first method for achieving the effect is referred to herein aspoint-source swarm flight. To create a sign, say consisting of theletter “C”, one needs about 5 satellites standing for points along thecurve, at say equal spacing Similarly, for the letter “I” one needs 3 to4 satellites. Of course, the more satellites per letter, the moreaccurate the display will be, see FIGS. 2 and 3. Once the relativepositioning is achieved, so as to be seen from earth, and for anappropriate duration, the satellites forming the letter or letters ofthe sign, will shine and continue orbiting earth. The timing iscarefully designed so as to create the effect above the desired area andat the desired time. Due to optical limitations, the time of displaywill usually be limited to the duration of dusk to dawn although certainextensions, using mirrors and lenses as described subsequently, mayachieve longer visibility.

It is noted that due to energy consumption, without using a lens andmirror extension, and considering the rapidness of overhead transit inLEO, the visibility period may be limited in duration to a few minutesat a time, followed by a recharging period. The energy is limited by thebattery size, and the battery size is limited by the weight limit of thenano satellite.

Logos and other complex signs may be displayable as discussed above, thelimiting factor being the number of satellites.

In an embodiment, an array of satellites may fly in a fixed array whereeach satellite is one pixel of the array. The letters or signs areformed by switching on the lights of the satellites whose pixels arerequired by the sign and leaving the others switched off. In this way,no shuffling around of the satellites is needed when the message ischanged.

Another technique is partial-showing, that is displaying the sign inportions. To illustrate this idea consider an array of six satellitesdisplaying the term “COKE”™ one letter at a time. First, the six displayC; Then they maneuver or change their lighting sequence to display O,then K, and finally E. This way, one can obtain better resolution forthe individual letters without having to increase the number ofsatellites. Another advantage is making do with a limited amount ofenergy per satellite. Suppose twelve satellites are to display “COKE”.Suppose they each have enough energy for twice participating in showinga letter. Then, the first six can display C, and then O. Then the othersix can display K and then E.

In a further embodiment, there may be provided an array of say 4×4 or5×5 satellites that selectively light up in patterns that display asubsequent letter each time.

A further method is referred to as the dropped elements method. Thismethod is similar to the point source swarm flight. The difference isthat satellites drop sub-satellites that are not self-maneuverable. Theydo however act as display elements, which are dropped into place by theactive maneuverable satellites and can be activated either via broadcastor by pre-determined timers. There are two kinds of such elements:recoverable and non-recoverable. The recoverable ones may be collectedat the end of the display period by the maneuverable satellites. Thenon-recoverable ones may typically self-destruct by decaying within afew years due to atmospheric drag, and burn in the atmosphere.Self-destruction may be quickened by providing acceleration towardsearth. It should be noted that the accuracy of the display here may beless than that of the point source swarm flight due to drift effects.

In one embodiment the dropped elements may simply be flares which arereleased from the satellites when in formation and which remain lituntil burned out and burned up.

A further method is the mother ship method. In both the point source anddropped element methods, certain resources may be exhausted or may fail.For example, fuel for maneuvering may run out, a satellite's controlelectronics or communication electronics or motors may malfunction,non-maneuverable elements may be lost, lamps may fail and the like. Onetemporary solution is launching a larger satellite that will hold suchresources and dispense them as required. An alternative is to start offwith extra resources in terms of redundant elements on individualsatellites or additional satellites, and to launch additional ones asneeded. But, because launch dates are not always predictable, this mayrender the system as non-operational at times. Putting these twotogether, a mother ship, that is a larger satellite with themaneuverability to service the nano satellites, plus occasionalreplenishment, may improve long term system reliability.

Reference is now made to FIG. 6, which is a simplified diagramillustrating two satellites 60 and 62 with a wire 64 between them and alamp 66. A further method, using the arrangement of FIG. 6 is known asthe wires method. The idea here is to improve the pixel resolutionavailable from a fixed number of satellites beyond the points availableon the satellites or sub-satellites themselves. Here the satellites areconnected via lightweight wires. These wires have sections that can belit so as to be viewable from earth, typically carrying their own LEDs.The placement technique is similar to the arrays used above, and energyis supplied from the wire-end-point satellites. The wires, if somewhatstiff, can also be used to maintain the structure of the array, so thatindividual satellites do not drift.

Reference is now made to FIG. 7. Shown schematically and not to scale,four satellites 70, 72, 74 and 76 hold a sheet 78 between them. A yetfurther method using the arrangement of FIG. 7 is the sheet method. Inthe sheet method a thin sheet of a large size (say, 1 km by 1 km) isdeployed via maneuverable satellites. As the sheet can be heavy (at 0.1gr per meter squared, a 1 km by 1 km sign would weigh 100 kg), a highlyporous structure is preferable. The sheet may be launched separately, asstand-alone cargo or with the mother ship satellites. The sheet mayeither have a message built in or may be a switchable matrix. Switchingmay involve switching particular lamps on and off, which serve to makeparts of the sheet visible and other parts invisible. New sheets can bedeployed as required. The technology for achieving this effect may bethat used for large solar panel deployment in space.

Reference is now made to FIG. 8, in which a first nanosatellite 80, inreality an array of satellites, focuses light from the sun using a lens82 onto a mirror 84 held by another satellite 86—again in reality anarray. The mirror reflects the light to the ground and thus provides theillumination event. The second satellite is on the dark side of earthbut the first satellite is in the path of the sun's rays, so that lightis directed to the ground by the second satellite to provideillumination for an event method is the lens and mirror method. In thismethod there are two arrays or swarms of satellites. Some satellites,“swarm L” possibly being or including the mother ship if there is oneare in sun-facing positions while others are in a dark area, hereinafter“swarm D”. Swarm-L satellites collect sunlight, focus it via lenses, anddirect it to the swarm D satellites. The swarm D satellites use mirrorsto directly display the focused light towards earth, as per the pointsource method, to create a continuous display of the sign. The same canbe done for the dropped element method. For the sheet method, the sheetitself may be blank and the swarm L satellites can project directly ontothe sheet, the sheet will illuminate at the respective points and theresult may be visible from earth.

The following describes certain system elements:

Formation: In order for the unaided eye to capture a pattern in the sky,it needs to be sufficiently large. A typical human eye can resolve 1minute of arc ( 1/60 deg.) At a distance of 600 km, this makes for 200 mresolution elements. Hence, at least 200 m should be the typicaldistance between satellites. Any letter of the alphabet can begraphically depicted using a 4×4 (or 5×5) array of points implying acluster of 16 (25) satellites spanning a region of approximately 1 km×1km.

Control System: The control system, for any of the abovementionedMethods, is in charge of receiving display missions from earth andcarrying them out. The control system may be pre-programmed, reside onone or more of the satellites, or on the mother ship (when there is one)or may reside on the ground, with instructions being relayed to one orall of the satellites.

As during most of the LEO orbit the satellites are not in sight of anyparticular ground station, it is necessary to at least partially run thecontrol system autonomously from space.

Energy: The energy to display the sign via intense light may be gatheredusing photovoltaic panels. The energy may be accumulated in a batteryand discharged in short duration bursts, that is of a few minutes, toachieve visibility from earth. This is also the case for sub-satellitesthat may receive energy from the satellites or independently.

Human scotopic vision is sensitive to less than 100 visible (say green)photons per second, or the light power of ˜3×10⁻¹⁰ erg s⁻¹. 3×10⁻¹⁷ W.These photons need be captured by the pupil of the human eye (the“reception area”). Given that the area of the human pupil in the dark isslightly less than 1 cm², this implies the unaided eye is able toidentify in darkness a light source that shines on the eye an energyflux of ˜5×10 ⁻¹⁰ erg s⁻¹ cm⁻² or 5×10⁻¹⁷ W cm⁻². Noting that theatmospheric transmittance of visible light is 50%-60%, but varying withlatitude, humidity and pollution, we may require the satellites to emitan energy flux F of 10⁻⁹ erg s⁻¹ cm⁻²=10⁻¹⁶ W cm⁻². Since the satellitesare at a distance of d=600 km above the ground (stable LEO orbit), andemit unbeamed light (in order to be visible over a large area on theground), the power P they are required to shine downwards (half spheresurface area is 2πd²) is P=2πd²F, which is just above 2×10⁷ erg s⁻¹, or2 Watts of light. A LED connected to the satellite battery could producea few Watts of light for a few minutes before the satellite moves out ofsight, and the battery can be recharged. At certain observing angles,reflective panels on board the satellite would further augment theelectrically produced luminosity.

Cubesat batteries may be able to light LEDs of several dozen Watts,which would make the signs all the more visible from Earth, over alarger area of ground, and allow for higher and thus more stable orbits.As LED technology improves brighter and longer lasting signs can bedisplayed from space.

The system using the array of satellites may produce display events.Given n display events to be shown, the events may be processed asfollows:

a. A naïve simulation mode in which, based on the expected positions andenergy contents of the satellites, a plan is produced and is assumed toperform perfectly with no deviations.

b. A realistic simulation mode in which, based upon system learntparameters, deviations are probabilistically introduced and a Trackingfunction applies corrections. The success probability is estimated andif larger than P% (P is expected to be 99%) the plan is certified andcarried out.

c. If the plan is not certified, an event is chosen and deleted fromcurrent consideration and the resulting sequence is re-considered. Thedeleted event may then be inserted in another batch, as required.

d. Next, a plan is produced and tracked in real time, in the naïve modein which everything is performed perfectly. In the realistic simulationmode, a module provides the simulated measurements. In an actualexecution, as displayed below, the performance is real and is carriedout after the plan is certified. The actual performance in effectreproduces the plan or a variation thereof in real time, based on theup-to-date state of the system.

Viewing Events Management

Reference is now made to FIG. 9, which illustrates a viewing eventsmanagement module 90 according to an embodiment of the presentinvention. The module is used in the naïve simulation 92, and then alsoin a realistic simulation 94 where system measurements are simulated,and in the real-time mode 96 to actually carry out the batch.

Some important variables and parameters are as follows:

Positions could be 3D or 2D (angles) time-dependent vectors in anon-rotating geocentric coordinate system.

pe—position of highest priority target for viewing display on earth.pcom—best estimated position of center-of-mass of satellite cluster.pi—position of the I'th satellite with respect to pcom, e.g. from GPS.ei—energy content in the I'th satellite, in ergs.vi—velocity vector of the I'th satellite with respect to coordinatesystem (in km/s).t—system time in microseconds.e.next—the next display event.ej—the j'th display event, ordered by display start time expressed insystem time units.Structure of a display event e:

A display event in the event module may be structured as follows:

1. e.start.position Event start position in system coordinates.

2. e.start.time Event start time.

3. e.start.time.boundary a pair of values indicating tolerance for starttime.

4. e.end.time.boundary a pair of values indicating tolerance for endtime.

5. e.end.position Event end position in system coordinates.

6. e.end.time Event end time.

7. e.special.effects Special effects (blink, spike, fade, . . . ).

8. Visibility level (L, M, H) at target viewing position.

9. e.ns Number of satellites to be employed.

10. For j=1 to ns, let pj be the position, relative to pcom of the j'thparticipating satellite. pcom should coincide initially withe.start.position, and at the end of the event with e.end.position. It isdesigned to provide the best visibility at pe.

11. e.me Maximum amount of energy units in ergs to be used.

12. e.start.2.position Secondary start position.

13. e.start.2.time Secondary start time.

14. e.end.2.position Secondary end position.

15. e.end.2.time Secondary end time.

16. e.start.2.time.boundary a pair of values indicating the tolerancefor start time.

17. e.end.time.2.boundary a pair of values indicating the tolerance forend time.

Parameters 11-17 are intended for use when the primary event can not becarried out, and a secondary one is loaded to the system for back up.More than one secondary set of parameters can be loaded to the system.

Structure of a command c to the satellite:

A command to a satellite may be structured as follows:

1. c.start.time—Command start time in system time.2. c.thrust—Command module, which activates thrusters. Includes nozzles,direction, and time of activation.3. expected start.position—expected position in system coordinate systembefore activation.4. expected end.position.after.activation—expected position in systemcoordinate system after activation.5. expected.energy.used—expected energy in ergs to be used.

In order to reach a desired position at a desired time, the satellitemay be issued a sequence of such commands.

Produce Flight Plan

A flight plan may be produced as follows:

input: A sequence e1, . . . , en of display events each with its fieldsaccording to the structure.Output: A sequence of instructions with the format above.

Method:

a. Estimate calculation time calc.time in microseconds;b. no =t; /* Now is current system time */c.c. /* The execution starting time for the events is estimated orcalculated thus: */d. Calculate position pi of all satellites i at est=now+2*calc.time;/*precaution */e. Handle events in sequence starting with el, let e be the currentlytreated event;

a. For each satellite i calculate a trajectory that will bring satellitei to the location indicated by e.start.position at time e.start.time;For each such trajectory, calculate the expected energy usage ui bysatellite i;

b. For some satellites such a trajectory will not be possible as theyare either engaged on other missions, that is marked as busy up to alater point in time than e.start.time, so they cannot participate, orthey may be unable to participate due to physical limitations, includingenergy and speed.

c. Choose a group G of ns satellites so that the remaining energycontent of a satellite in G after reaching e.start.position is as largeas possible (maximal); /* The heuristic used is that all satellitesreaching the start of display position have sufficient energy toactually make the display */

d. Verify that the group satellites have sufficient energy to actuallymake the display;

e. If not then do the following:

-   -   i. If e is marked “not performed” issue a notification that e        will not be executed in this batch and delete e from the        sequence;    -   ii. Otherwise, e is marked “not performed” and will be        considered with its secondary display option later on, it is        simply added to the end of the sequence and the secondary        attributes replace the primary ones and the next event is        considered;

f. For ease of notation the satellites in G are sj, j=1 . . . e.ns;

g. For satellite j, issue a sequence of commands to satellite j whoseexecution starts at est so that at time e.start.time the sequence willposition j in the viewing circle position j where the circle's centercoincides at that time with e.start.position;

h. Use a method for a display action, herein referred to as‘generateCircleViewing’ that outputs a sequence of commands so that forthe duration of e satellite j will perform the display functionalityrequired of the j′th satellite in the group performing e. Update theexpected energy level after performing the event; indicate that thesatellite j is busy until e.end.time+DELTA; /* DELTA is a precautionsystem constant, about 30 seconds */

i. Delete e from the sequence;

j. /* At this point commands are issued to perform event e */

k. At this point /* real data refers to calculated results in simulationmode, to system generated simulation data in simulation mode and toactual data in real time execution mode */:

-   -   i. Update energy content now for all satellites; /* real data */    -   ii. Update the position of all satellites; /* real data */iii.    -   iii. Update energy content for busy satellites at the end of        their busy period;    -   iv. Update expected location for busy satellites at end of their        busy period;    -   v. Delete any malfunctioning satellites temporarily from        consideration.    -   vi. If the sequence is empty, then move to tracking mode,        verifying the plan is properly executed and alerting ground        control to malfunctions;    -   vii. Otherwise, handle the next event in the sequence;

Tracking

Tracking compares the actual position and energy levels of satellites atfrequent time intervals. In between tracking measurements, standardorbit propagation codes can be used (e.g., STK). Note that the energycontent may increase due to photovoltaic energy production or due toother possibilities of energy infusion such as direct laser charging.

When tracking a busy satellite, based on actual positions, correctingcommands may need to be issued so as to carry out the relevant event eas close as possible to the plan. For example there may be a need toaccelerate members of a group displaying an event e, there may be a needto reschedule the event start time if indicated as possible, that is ifthere is a possible time that lies within the specified time boundarye.start.boundary, and the same may apply for the end time e.end.time.

Aside from issuing such commands, Tracking may involve keeping adetailed system log of all commands issued and all statuses recorded.Red alert events, such as a satellite malfunctioning, are flagged inreal time to the tracking management team.

Altering

During execution there may be a need to alter the plan. For example, adisplay for an outdoor event is canceled due to weather conditions. Forsimplicity, assume cancelled events are not currently executing ones,namely they are to be executed in the future and there is sufficienttime to calculate a new plan. The idea is to delete from execution allcommands on behalf of non-executed events from the plan. Then, theevents are reintroduced and a new plan for them is constructed based onthe current positions and the positions at the end of busy times.

Use of Non-visible Light

The above embodiments have been described as using visible light, andallowing naked eye viewing. However it is also possible to provide adisplay for only visible by telescope and also to provide a display thatis based on non-visible wavelengths of light such as infra-red, andparticularly the near-infra-red. Generally UV light is highly absorbedin the atmosphere. A display that is based on non-visible light, forexample infrared, may (a) somewhat reduce energy consumption, and (b)more importantly, may eliminate light pollution in space which may besubject to laws or regulations. In the case of non-visible light specialdevices may be provided to view the display.

Use of UAV's

The above embodiments have related to satellites in orbit. However,light patterns could also be created using a formation or a swarm ofcraft flying in the atmosphere such as unmanned aerial vehicles (UAVs).UAV's may be subject to different regulation regimes from satellites,and the formations may be located closer to the ground, thus allowingthe same illumination for less energy output. UAV's can also usenon-visible wavelengths and unlike with satellites, UV light can reachthe ground without being absorbed. The UAVs are typically unmanned andmay be controlled from the ground. Alternatively a manned aircraft maybe used to control a swarm of UAV's. Control may be direct, or in thecase of the manned aircraft may be implicit, by having the UAVs followthe cues of the leader airplane.

It is expected that during the life of a patent maturing from thisapplication many relevant nano satellites and nano-satellite controlsystems will be developed and the scopes of the corresponding terms areintended to include all such new technologies a priori.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment, and the abovedescription is to be construed as if this combination were explicitlywritten. Conversely, various features of the invention, which are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any suitable subcombination or as suitable inany other described embodiment of the invention, and the abovedescription is to be construed as if these separate embodiments wereexplicitly written. Certain features described in the context of variousembodiments are not to be considered essential features of thoseembodiments, unless the embodiment is inoperative without thoseelements.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

What is claimed is:
 1. A nanosatellite carrying an illumination element,the illumination element configured for sustained external illumination.2. The nanosatellite of claim 1, wherein said illumination elementcomprises a light emitting diode.
 3. The nanosatellite of claim 2,comprising battery power sufficient to illuminate said lamp for at leasta minute, and photovoltaic recharging ability.
 4. The nanosatellite ofclaim 1, configured with control ability to position itself with othernanosatellites in a predetermined array.
 5. The nanosatellite of claim4, configured with control ability to operate said lamp in coordinationwith said other nanosatellites, thereby to provide an illumination eventvisible from the ground.
 6. The nanosatellite of claim 1, havingdirectional capability to point said illumination element towardsground.
 7. The nanosatellite of claim 1, wherein said illuminationelement is releasable.
 8. The nanosatellite of claim 7, configured withcontrol ability to position itself with other nanosatellites in anarray, and to release said illumination element in coordination withsaid other nanosatellites, thereby to provide an illumination eventvisible from the ground.
 9. The nanosatellite of claim 1, with controlability to position itself with other nanosatellites in an array andcarrying a wire connected to at least one other of said nanosatellitesin said array, at least one additional illumination element beinglocated on said wire.
 10. The nanosatellite of claim 1, with controlability to position itself with other nanosatellites in an array whileholding a sheet connected to others of said nanosatellites in saidarray.
 11. The nanosatellite of claim 1, wherein said illuminationelement is a mirror, said mirror being directable to deflect focusedlight from another satellite source at the ground.
 12. The nanosatelliteof claim 1, wherein said illumination element is an element illuminatingin a frequency range outside of a visible part of the electromagneticspectrum.
 13. A ground-based satellite control system configured tocontrol a first array of nanosatellites in low earth orbit to produce anillumination event visible at a predetermined ground location.
 14. Theground-based satellite control system of claim 13, wherein said firstarray comprises at least one matrix of illumination positions, saidmatrix sized such that controllable switching of said illuminationpositions forms letters visible from said predetermined ground location.15. The ground-based satellite control system of claim 13, configured toswitch said illumination positions to change said letters during saidillumination event.
 16. The ground-based satellite control system ofclaim 13, configured to control a second array of nanosatellites toshine focused light onto said first array for redirection by said firstarray.
 17. The ground-based satellite control system of claim 13,configured to schedule said illumination event along with otherillumination events in accordance with available battery power on boardsaid nanosatellites and in accordance with recharging opportunities ofan orbit of said nanosatellites.
 18. The ground-based satellite controlsystem of claim 13 configured to add additional available nanosatellitesto said first array according to requirements of an illumination event.19. The ground-based satellite control system of claim 13, configured tocontrol an illumination event to fit in with a duration during whichsaid first array is visible over said predetermined ground location. 20.The ground based satellite control system of claim 13, wherein saidillumination event is an event illuminating in a frequency range outsideof a visible part of the electromagnetic spectrum.
 21. A method ofcontrolling nanosatellites comprising: arranging said nanosatellites ina first array in low earth orbit; and operating illumination elements inassociation with said nanosatellites to produce an illumination eventvisible at a predetermined ground location.
 22. The method ofcontrolling nanosatellites of claim 21, wherein said first arraycomprises at least one matrix of illumination positions, the methodcomprising sizing the matrix such that controllable switching of saidillumination positions forms letters visible from said predeterminedground location.
 23. The method of controlling nanosatellites of claim21, comprising switching said illumination positions to change saidletters during said illumination event.
 24. The method of controllingnanosatellites of claim 21, comprising controlling a second array ofnanosatellites to shine focused light onto said first array forredirection by said first array.
 25. The method of controllingnanosatellites of claim 21, comprising scheduling said illuminationevent along with other illumination events in accordance with availablebattery power on board said nanosatellites and in accordance withrecharging opportunities of an orbit of said nanosatellites.
 26. Themethod of controlling nanosatellites of claim 23, comprising addingadditional available nanosatellites to said first array according torequirements of an illumination event.
 27. The method of controllingnanosatellites of claim 21, comprising controlling an illumination eventto fit in with a duration during which said first array is visible oversaid predetermined ground location.
 28. The method of controllingnanosatellites of claim 21, comprising keeping a mother satellite inreserve and using said mother satellite to carry out maintenance on saidfirst array of nanosatellites when needed.
 29. The method of controllingnanosatellites of claim 21, wherein said illumination event is one of aseries of illumination events, the method comprising scheduling saidillumination event according to time frames provided for eachillumination event in said series.
 30. The method of controllingnanosatellites of claim 21, wherein said illumination event is an eventilluminating in a frequency range outside of a visible part of theelectromagnetic spectrum.
 31. A method of controlling unmanned aerialvehicles comprising: arranging said unmanned aerial vehicles in a flyingformation; and operating illumination elements in association with saidunmanned aerial vehicles to produce an illumination event detectable ata predetermined ground location.
 32. A control system configured tocontrol a first array of unmanned aerial vehicles in flight to producean illumination event detectable at a predetermined ground location. 33.The control system of claim 32, being ground-based.
 34. The controlsystem of claim 32, being air-based.